>> Do you have independent home switches on the slaved axes (allows correction of any skew/racking)?
Umm... nope. I'm kind of learning this stuff on the go. If I have skew or racking I dont know about it!!
The reason I asked is because if you don't, as you have confirmed, you could get away without a slaved axs: just put two step wires and two direction wires from your BoB to each X-stepper controller. Effectively you are splitting the output from Mach.
The beauty of the slaving and independent home switches is if one side gets out of synch. with the other (so the gantry will skew or rack), you can home the machine and when one side touches its switch, it will stop and wait for the other side to touch (thus deskewing). Once both home switches are triggered, then the axis numbers are changed.